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Genetics of Scrapie
Scrapie is a 100% fatal, degenerative disease affecting the central nervous
system of sheep and goats. The disease has been reported in countries throughout
the world with few notable exceptions (Australia and New Zealand). The first
case of scrapie was discovered in the United States in 1947. The current incidence
is 2/10 of 1 percent or 1 in 500 U.S. sheep. The incidence of scrapie in the
U.S. goat population is not currently known.
While bovine spongiform encephalopathy (BSE or “mad cow disease”)
is believed to be caused by the consumption of contaminated feed (meat and bone
meal), scrapie is transmitted during lambing when lambs come into contact with
infected placenta and birth fluids from infected ewes . Rams can get scrapie,
but are not known to transmit scrapie.
Scrapie is NOT caused by genetics, but the genetic make-up (DNA) of an animal
determines whether it will get scrapie if it is exposed to the infective agent.
In other words, if a genetically susceptible lamb is exposed to a scrapie-infected
placenta, it will develop scrapie. It takes from 2 to 5 years after exposure
for an animal to show clinical signs of scrapie. If a genetically resistant
lamb is exposed to a scrapie-infected placenta, it will not develop scrapie.
No resistant genotypes have been identified in goats.
How the Genetics Work
Sheep have one pair of genes that affects scrapie susceptibility. The pair of
genes are known as PRNP genes (PrioN Protein genes). Each sheep has two copies
(one pair) of the PRNP – one copy from each parent. PRNP exists in all
animals with small differences between species. All genes are made up of codons.
Each codon instructs cells to put a specific amino acid at a particular location
when building a protein molecule. The prion protein molecule (produced by PRNP
gene) has 254 amino acids. The locations of the 254 amino acids are numbered
1 through 254. In the PRNP gene, three codons affect scrapie susceptibility:
171, 154, and 136.
Codon 171 is the major determinant of scrapie susceptibility in the U.S. Codon
136 affects susceptibility in sheep exposed to some scrapie types. Codon 154
plays a minor role and is often not used in the U.S. Codon 136 programs for
the amino acids Valine (V) and Alanine (A). Codon 171 programs for the amino
acids Glutamine (Q), Arginine (R), Histidine (H), or Lysine (K). Q, H, and K
are considered to have the same susceptibility and are reported as Q by most
labs.
The genotypes of sheep in the U.S. are written in two ways: 1) letters of the
amino acids, AA QR, AV RR, etc.; or 2) Codon number followed by the corresponding
amino acids: 171QR, 171RR, etc.
Ewe Genotype |
Ram Genotype |
AQ |
VQ |
AR |
AQ |
AA
QQ |
AV
QQ |
AA QR |
VQ |
AV
QQ |
VV
QQ |
AV QR |
AR |
AA QR |
AV QR |
AA RR |
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Highly susceptible |
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Rarely susceptible |
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Resistant |
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Genotyping (DNA testing) can be used to determine a sheep’s susceptibility
to scrapie. A simple blood (or tissue) test is all that is required. A sheep’s
genotype never changes so only one test should be needed. Genotype can be determined
at any age. It is important to note that genotyping only measures susceptibility
to scrapie, not whether the animal has scrapie.
Producers should consider using scrapie genetics (genotyping) as a management
tool if:
• They have a breed in which scrapie is prevalent and they have purchased
ewes of unknown scrapie status.
• They have purchased ewes from an infected flock.
• They have observed signs of scrapie in their flocks in the past.
• Customers are requesting scrapie-resistant breeding stock.
• They wish to provide scrapie-resistant breeding stock to their customers.
• They are forward-thinking!
Scrapie Genotypes
AA RR - resistant
AA QR - rarely susceptible
AV QR - much less susceptible
AA QQ - highly susceptible
AV QQ - highly susceptible
VV QQ - highly susceptible |
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Source: Scrapie Control: Genotyping – A New Tool For
Producers. A PowerPoint presentation produced by the National Scrapie Education
Initiative (available online at http://www.animalagriculture.org/scrapie).
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